1. Field of the Invention
The present invention relates to surface acoustic wave devices and communication apparatuses including such surface acoustic wave devices. More particularly, the present invention relates to a surface acoustic wave device that is advantageously used as a surface acoustic wave branching filter and to a communication apparatus including such a surface acoustic wave device.
2. Description of the Related Art
Surface acoustic wave devices (hereinafter referred to as xe2x80x9cSAW devicesxe2x80x9d) include a surface acoustic wave element utilizing surface acoustic waves (SAWs) that propagate along the surface of a piezoelectric element, and are used in delay lines, filters, resonators, and other apparatuses. Since SAWs have a shorter wavelength than electromagnetic waves, they offer advantages in that, for example, SAW devices can be easily miniaturized. Thus, in the field of portable telephones and other electronic apparatuses, SAW devices are often used as filters in radio frequency circuits.
In recent years, particularly in the field of mobile communication, including portable telephones, there has been an increasing demand for further miniaturization and lower profile. Thus, SAW devices have also been required to be reduced in size and height. In particular, a portable telephone performs both transmission and reception in different frequency bands with a single antenna. Therefore, the demands on the use of SAW devices as branching filters are becoming greater.
Examples of technologies for such branching filters include a SAW branching filter disclosed in Japanese Unexamined Patent Application Publication No. 5-167388. This technology uses a plurality of SAW resonators (hereinafter simply referred to as xe2x80x9cresonatorsxe2x80x9d), as shown in FIG. 10, to constitute two SAW ladder filters (hereinafter may be referred to as xe2x80x9cfiltersxe2x80x9d). The filters are connected in parallel to define a branching filter.
More specifically, the branching filter uses ladder filters Fi and Fii having a configuration in which series resonators S and parallel resonators P are alternately connected, and the filters Fi and Fii are connected in parallel to a common terminal T0. Among the resonators S and P that constitute the filters Fi and Fii, resonators Si and Sii that are directly connected to the common terminal T0 are connected in series. When the filters Fi and Fii are compared, the filter Fii has a relatively high bandpass frequency, and is used as a reception filter when used in a communication apparatus. On the other hand, the filter Fi is used as a transmission filter.
As described above, with the branching filter being constituted by two SAW filters, and with respect to the filter Fi, the following filter characteristic (an impedance characteristic) is required. That is, the filter Fi needs to have an impedance that is close to the impedance of the entire circuit in the passband of the filter Fi, and needs to have an impedance that is significantly larger than the impedance of the entire circuit in a rejection band that is a passband of the filter Fii. When typical transversal SAW filters are used, it is not easy to achieve such a filter characteristic, resulting in a complicated circuit configuration of the entire branching circuit.
In contrast, the technology of Publication No. 5-167388 discussed above uses series resonators in which the resonators Si and Sii that are in closest proximity to the common terminal T0 are connected in series. Thus, the series resonators allow matching of impedance characteristics of the filters. The series resonators can also be used for phase adjustment in the configuration of the entire branching filter. Consequently, the impedance in the rejection band outside the range of the passband becomes quite high relative to the circuit impedance, thus making it possible to achieve the impedance characteristic described above.
In addition, Japanese Unexamined Patent Application Publication No. 11-068512 discloses a branching filter that uses SAW filters having a configuration other than a ladder filter. This technology utilizes, in the same manner as Publication No. 5-167388 discussed above, series resonators as elements for phase adjustment for a branching filter configuration.
In the case of a configuration in which SAW ladder filters are used to constitute a branching filter as discussed above, when resonators that are directly connected to a common terminal are connected in series in each SAW filter, the resonators (series resonators) can function as elements for phase adjustment. The series-connected resonators can also be used for phase adjustment in a bandpass filter other than a SAW ladder filter.
In order to prevent the deterioration of the impedance characteristics of each SAW filter, the impedance and frequency of the series resonators are critical parameters that affect the characteristics of the entire branching filter. Thus, in the technology disclosed in Publication No. 11-068512, the frequency of the series resonators, and the resonant frequency in particular are designed to match the center frequency of the passband of the SAW filter. This makes it possible to effectively prevent a decrease in the impedance characteristics of the SAW filter including the series resonators.
In the technology of Publication No. 5-167388, the resonant frequency of the series resonators is not particularly limited. With a ladder filter, however, in order to prevent a decrease in the impedance characteristics, it is still preferable to match the resonant frequency of the series resonators to the center frequency of the passband of the SAW filter including the series resonators.
In the technologies of the related art, when the resonant frequency is matched to the center frequency of the passband of the SAW filter under the condition that the impedance of the series resonators is set to be high for phase adjustment, this causes a problem in that the passband width is reduced.
That is, since the series resonators are used for phase adjustment, there is a need to restrict the impedance to a high range. Consequently, changes in the impedance of the resonators become considerably sharp relative to changes in the frequency. In particular, the frequency dependence of the impedance is maximized between the resonant frequency and the anti-resonant frequency.
When the resonant frequency of the series resonators is set to be the same as the center frequency of the passband of the SAW filter, there is no particular problem in a frequency band lower than the center of the passband since the impedance of the series resonators is low. In contrast, in a frequency band higher than the center of the passband, a sharp change in the impedance causes the impedance of the series resonators to become high. This results in an unsatisfactory matching in the high band, so that the high band, which is supposed to be a pass band, effectively becomes a rejection band (elimination band), thereby reducing the passband width.
When the band width ratio is small, even matching the resonant frequency of the series resonators to the center frequency of the passband does not substantially reduce the passband width. However, when the band width ratio is large, there is a problem in that sufficient matching between the SAW filters cannot be achieved.
In order to overcome the problems described above, preferred embodiments of the present invention provide a SAW device, constituted by a plurality of SAW filters, that achieves an improved matching characteristic and that prevents a reduction in passband even in the case of a large band width ratio of each SAW filter when used as a branching filter. In addition, preferred embodiments of the present invention provide a communication apparatus including such a novel SAW device.
According to a preferred embodiment of the present invention, a SAW device includes a first SAW filter that has a relatively low bandpass frequency, a second SAW filter that has a relatively high bandpass frequency and that includes a plurality of SAW resonators, and a common terminal to which the first and second filters are connected. One of the SAW resonators which is connected in closest proximity to the common terminal is connected in series and has a resonant frequency that is higher than the center frequency of the passband of the second surface wave filter.
In the above configuration, among the plurality of SAW resonators (resonators) included in the second SAW filter (second filter), the resonant frequency of the resonator (junction-side resonator) adjacent to the common terminal, i.e., adjacent to a portion to which both the filters are connected, is higher than the center frequency of the passband of the second filter. Thus, the condition f0 less than fsr is satisfied, where f0 is the center frequency and fsr is the resonant frequency of the junction-side resonator.
Typically, a SAW device having a configuration in which two SAW filters are connected to a common terminal is preferably used. In such a configuration, the junction-side resonator is also used as a matching element. For this reason, however, it is conventionally difficult to achieve satisfactory matching at a frequency higher than the resonant frequency, i.e., in a high band, so that the passband width is reduced.
In contrast, in the SAW device having the configuration according to preferred embodiments of the present invention, the center frequency f0 of the passband of the second filter and the resonant frequency fsr of the junction-side resonator satisfies the condition f0 less than fsr. This allows a reduction in the frequency dependence of the impedance in the passband. The present invention, therefore, can provide a branching filter that prevents the reduction of the passband.
According to another preferred embodiment of the present invention, a SAW device includes a first SAW filter that has a relatively low bandpass frequency, a second SAW filter that has a relatively high bandpass frequency and that includes a plurality of SAW resonators, and a common terminal to which the first and second filters are connected. One of the SAW resonators which is connected in closest proximity to the common terminal is connected in series and has a resonant frequency fsr that is within the range given by the expression:
f0+BWxc3x970.2xe2x89xa6fsrxe2x89xa6f0+BWxc3x970.7xe2x80x83xe2x80x83(1)
where f0 is the center frequency of the passband and BW is the passband width of the second SAW filter.
Preferably, the resonant frequency fsr is within the range given by the expression:
f0+BWxc3x970.3xe2x89xa6fsrxe2x89xa6f0+BWxc3x970.6xe2x80x83xe2x80x83(2)
In the above configuration, the resonant frequency of the junction-side resonator included in the second filter is set in the range defined by expression (1), and preferably, within the range defined by expression (2). Consequently, the resonant frequency is further optimized, so that not only the passband can be prevented from being reduced, but also can be further broadened. This configuration, therefore, can provide a branching filter having a broader passband.
Preferably, the SAW filters are ladder-type filters.
In the above configuration, since the filters are of a ladder type, it is possible to ensure broadening of the passband and to provide a branching filter having improved performance.
The SAW device may further include matching device connected adjacent to the common terminal.
In the above-described configuration, the junction-side resonator functions as a phase-adjusting element, and the inclusion of the matching device can achieve satisfactory matching of the filters.
According to still another preferred embodiment of the present invention, a communication apparatus includes a SAW device having the configuration according to preferred embodiments described above. The SAW device is used as a branching filter.
In the above configuration, since the SAW device of various preferred embodiments of the present invention is used as a branching filter, it is possible to provide a communication apparatus having a highly favorable transmission and reception capability.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.